Sleeping pads for outdoor use have comfort requirements similar to those of indoor beds and cushions. They also have added requirements for durability and portability. The tradeoffs which exist between these three general requirements and the materials available for construction have determined the evolution and effectiveness of ground pads developed to date.
Lacking significant thickness or weight constraints, most bed mattresses are made of several layers of various foams, textiles and spring assemblies. By varying the compliance and resiliency of each layer, indoor mattresses can be designed to meet virtually all user requirements.
For ground pads, direct use of indoor mattress designs are infeasible due to the requirement that the ground pad be easily transported by the individual. However, a ground pad needs to have enough compliance to feel comfortable, but not so much that the individual user "bottoms out" on the ground. One method of achieving compliance is by increasing the thickness of the pad, but only at the sacrifice of increasing the stored volume and weight.
Ground pads also have special comfort-related requirements which are unique to their use environment. Thermal loss due to conduction, convection and radiation are important factors, especially due to the fact that most ground pads are thinner and rest on colder surfaces than indoor mattresses. Because they are often used in wet environments, resistance to moisture absorption is also a key consideration in the design of a ground pad.
Relatively early, ground pads were made of natural rubber foams, which were both elastic and could be molded to intricate shapes The natural foam rubber ground pad offered new features which were only partially exploited because of the relatively low compliance of natural rubber. However, introduction of latex foam rubber offered a further comfort breakthrough for mattresses because of its softness, resiliency and resistance to fatigue.
The natural rubber and latex foam rubbers, as well as urethane foams, incorporate an open cell structure. That is the rubber is formed by a number of cells which are in communication with each other via openings in the cells. Resistance to compression of these foams is mainly due to the structural support provided by the cellular walls As the open cell foam is compressed, the air within the cells is displaced into the atmosphere.
An open cell structure has several additional disadvantages. First, it promotes the absorption of water from wet supporting surfaces into the structure, much like a household sponge (which is commonly made from an open cell foam material) increasing the pad's weight and promoting moisture transfer to the user's sleeping bag. As a result, many of the open cell foam ground pads have an outer water impervious cover to prevent their water absorption. Second, the open cell structure is also less effective as a thermal insulator due to intercellular openings which facilitate heat transfer. Also, open cell foams allow water vapor to pass through the foam and to condense on an underlying colder surface such as the ground or on the bottom surface of the foam pad, causing the foam to get wet and reduce its insulation value.
A further advance in ground pad design was achieved by the development of several soft, low density, closed cell polymeric foams such as a vinyl-nitrile copolymer known as Ensolite. Reductions in weight and cost of closed cell foam ground pads were achieved through the use of a foamed copolymer of ethylene and vinyl acetate, also known as ethylene-vinyl acetate (EVA). When used as a ground pad material, EVA foam appears to provide the best balance over all other closed cell foams in terms of economy, weight, durability and stored volume.
In addition, not only is the closed cell structure resistant to water absorption, but it also reduces heat loss. This is primarily due to the individual cellular pockets which are essentially sealed and contain therein trapped gases. The presence of the trapped gases, however, tends to make the closed cell pad less compliant than the open cell pad, because the gases must be compressed when the foam is loaded.
A number of support mattresses and pads made of foamed material and the like, have been disclosed. For example, support devices which are configured to be flexible along a specific axis of orientation are disclosed in U.S. Pat. No. 4,370,767 (beach mat) by Fraser; U.S. Pat. No. 4,275,473 (buoyant mattress) by Poirier; and U.S. Pat. No. 4,399,574 by Shuman (foam mattress pad).
Other support apparati which have specific geometries for increasing compliance were disclosed in U.S. Pat. No. 4,110,881 by Thompson, where the surface of a mattress is slotted so it may not be put in tension; U.S. Pat. No. 4,383,342 by Forster, where a plurality of upstanding flexible ribs are tilted at selected angles to achieve a traction force; and U.S. Pat. No. 3,197,357 by Schulpen, where an open cell or closed cell foam pad includes corrugations on at least one surface to increase compressability and compliance. Also disclosed are U.S. Pat. No. 2.194,364 by Minor, which shows a sponge rubber carpet pad which has ridges and valleys which are alleged to entrain air as a cushioning agent; U.S. Pat. No. 2,751,609 by Oesterling, which discloses an insulating pad formed by a plurality of easily compressible blocks secured to a backing sheet; U.S. Pat. No. 3,016,317 by Brunner, which discloses a closed cell resilient mat which has a number of lengthwise and transverse grooves which are made by a thermoforming process; and U.S. Pat. No. 3,814,030 by Morgan, which shows a mesh-like support member which is formed in a corrugated manner by thermoforming, injection molding, extrusion or the like.
In addition to the aforementioned disclosures, a number of multilayered support apparati have been disclosed, such as U.S. Pat. No. 839,834 by Gray (ribbed surfaces oriented at right angles); U.S. Pat. No. 2,953,195 by Turck (opposing sawtooth configured members separated by an inner planar layer); U.S. Pat. No. 4,450,193 by Staebler (mat assembly); U.S. Pat. No. 4.476,594 by McLeod (reversible mattress); and U.S. Pat. No. 4,574,101 (exercise mat containing internal air chambers).
Also disclosed is a support pad having an exterior cover in U.S. Pat. No. 4,329,747 by Russell and an inflatable cushion in U.S. Pat. No. 4,076,872 by Lewicki.